Aircraft with two or more engines positioned on opposite sides of the longitudinal axis of the aircraft have a tendency to turn if the torque generated by such engines is unequal. In practice, a mismatch in torque is unavoidable. Primarily this is due to the fact that at equal power settings similar engines will differ in power by 1-3%. In addition, oppositely positioned engines frequently are slightly misaligned resulting in turning moments on the aircraft.
During flight, a pilot will set equal power for both engines and position the aircraft to fly a predetermined course. Flight control will maintain this course and overcome the unequal torque of the engines by use of a continuous rudder correction. However, such rudder correction results in a degradation of flight efficiency by creating a drag on the aircraft.